Borrowings - Long-term Borrowings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Borrowings [Line Items]
Fixed rates	$ 1,158,216	$ 1,161,514
Agreements to repurchase securities	1,899,281	1,400,000
Other	20,902	23,011
Long-term borrowings	10,653,858	9,006,959
Variable Rate Notes Due 2016 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	300,000	300,000
Variable Rate Term Loans Due 2017 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	550,000	550,000
1.25% Due 2017 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	499,984	499,969
1.40% Due 2017 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	749,851	749,756
1.45% Due 2018 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	503,527	503,118
2.25% Due 2019 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	648,628	648,243
2.30% Due 2019 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	749,219	748,965
2.10% Due 2020 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	749,650
2.90% Due 2025 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	749,236
8.5% due 2018 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	213,417	218,883
6.625% due 2017 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	419,800	428,627
5.585% due 2020, Variable Rate Commenced 2015 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	409,361	400,846
5.629% due 2021, Variable Rate Commencing 2016 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	518,797	538,961
M&T Capital Trust I - 8.234%, due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities		154,640
M&T Capital Trust II - 8.277%, due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities		103,093
M&T Capital Trust III - 9.25%, due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities		65,784
BSB Capital Trust I - 8.125%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	15,635	15,612
Provident Trust I - 8.29%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	25,817	25,405
Southern Financial Statutory Trust I - 10.60%, due 2030 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	6,583	6,550
First Maryland Capital I - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	145,717	145,179
First Maryland Capital II - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	147,291	146,627
Allfirst Asset Trust - due 2029 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	96,349	96,204
BSB Capital Trust III - due 2033 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	15,464	15,464
Provident Statutory Trust III - Due 2033 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	53,244	52,692
Southern Financial Capital Trust III - due 2033 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	$ 7,889	$ 7,816